The invention relates to a method and apparatus for determining the composition of glass of unknown composition, and for sorting glass according to composition.
The environmental regulations of the U.S. and other nations currently classify the glass and other components of television tubes and color computer monitors as hazardous waste. Due to this classification, the direct landfilling of tubes which are found to be defective or which are broken in manufacturing, is no longer permitted. The disposal of this material requires a stabilization treatment before it can be legally landfilled. Elsewhere in the world there are more stringent restrictions on the disposal of these materials.
To further complicate the disposal issue, discussions have been started at various regulatory agencies concerning the disposal of cathode ray tubes (CRTs) from television sets and color computer monitors at the end of their useful life. There are approximately 250,000,000 television sets and 60,000,000 computer monitors in use in the United States. The disposal of 5% of the television sets and 10% of the computer monitors would represent an estimated 130,000 tons of CRT glass annually. Landfill disposal is the least desirable option for both the generator and the municipal planners.
The environmental issue which is of most concern relative to the disposal of the CRT glass is the potential for lead to be released from the materials. The most vulnerable portion of the CRT tube is the frit which is used to join the face plate (panel) to the funnel. This frit is comprised largely of lead oxide, which is slightly soluble in weak acids and thus vulnerable to leaching and migration into ground waters when buried in a landfill.
The CRT glass contains substantial quantities of lead oxide (PbO). Generally, lead oxide in this form is inert and environmentally stable. If the lead bearing glass is broken up or ground up into a fine powder and exposed to an acidic environment, such as might exist in a landfill, small quantities of lead can be leached out.
In addition, the interior coatings of older television CRTs contain high levels of cadmium compounds which are toxic. The landfilling of these CRTs can lead to the leaching of the cadmium compounds and their release into the ground water. The current manufacturing processes utilize different coatings which are not known to be toxic or carcinogenic; however, toxicological studies are being performed on a continuous bases to insure the safety of those who come in contact with the materials.
A perception exists that large amounts of lead would potentially be disposed of by the landfilling of electronic assemblies. It is therefore desirable to avoid landfilling these materials in order to lower costs, reduce the environmental risk and to assure the public safety.
The avoidance of landfill disposal should preferably encompass the total use of the glassy materials of the television CRTs, defective tubes and debris from manufacturing. Much of the current practice involves the reformulation of these glasses into raw materials. The raw materials are then used to make new products including electrical resistance glass; decorative glass; glass for the absorption of low-level radiation from low-level radioactive wastes; glass for retroreflective use and high density glass. In these cases, the products are unique in their function or in their substitution for other glasses. In all cases, the products have greater value in us or are lower priced substitutes forexisting products manufactured from non-recycled glass. A competitive advantage exists in the use of waste streams as raw materials for the manufacture of these unique products.
While the reformulation of CRT derived cullet into some new products does not require careful control of cullet formulation, if the glasses could be successfully separated by formulation from waste stream, then they would be more easily reformulated into useful products. By identifying the glass composition, it would be easier to channel cullet to the appropriate process for inclusion as a raw material in the production of a final salable product.
In addition to the utilization of the separated glasses as raw materials in the manufacture of secondary products, a new market has been established for the resale of the identified glass back to the CRT manufacturers for remelting. The criteria for this type of separation and recycling are very stringent and the separation requires highly accurate identification of the glass composition. CRT glass is a high quality optical glass. In order to manufacture this glass to the required quality standards, it is necessary to insure that the composition remain stable. The use of clean cullet is desirable because it allows the generator to avoid the cost of landfilling and reduces the melting costs. The remelting of cullet requires significantly less energy than melting raw materials to form the same glass. Compositional upsets can occur if the cullet is misidentified. This situation can result in large monetary losses on the order of $300/ton plus the cost of any processing.
The key component to the successful use of the discarded glass is accurate and reliable sorting. If the sorting is done accurately, the glass plant can save as much as $170/ton for materials and processing. In addition, in order to be commercially viable, sorting should be fast.
It would therefore be desirable to provide a means for accurately, reliably and quickly sorting CRT derived glass cullet according to glass composition.